Conventional paper sack constructions suitable for holding and storing of powdery material, such as cement or other building materials, chemicals, food, animal feed and pet food usually has a sack wall that consists of two or three paper plies in order increase the weight which can be held by the sack. Further, since a material such as cement is sensitive to deterioration caused by e.g. moisture penetration during storage, conventional paper sacks are also provided with a moisture barrier film, e.g. of polyethylene (PE), between the plies. The intermediate film functions as a moisture or water barrier and decreases atmospheric vapour penetration through the sack plies into the sack interior.
However, during filling of the sack, the deaeration rate, i.e. the speed at which air inside the sack is transported out from the sack, is often the factor that limits the rate at which the sack can be filled. Air trapped in the sack may lead to under-weight packs, sack rupture and problems when sacks are stacked for transportation. The use of a moisture or water barrier film as an intermediate layers tends to severely reduce the deaeration rate of a sack during filling, since most intermediate moisture barrier films used are highly impermeable to air. Therefore, the moisture barrier film may be provided with slits or perforations to facilitate deaeration of the sack. However, a slitted moisture barrier film obviously decreases the moisture barrier properties of the film, which may lead to moisture penetration.